Gas turbines have been utilized for some time as cost effective and efficient means for generation of electric power and steam. However, the temperatures at which turbines are operated produce unacceptably high amounts of nitrogen oxides (NO.sub.x). During the early seventies, efforts were made to annex catalytic combustors to gas turbines for the purpose of preventing NO.sub.x formation in the combustion process.
Efficient, modern turbines must operate at about 2500.degree. F. and up to as high as 2700.degree. F. for good thermal efficiency. At these temperatures, most metallic catalytic support materials are subject to weakening by grain growth, oxidation, cyclic failure, creep and loss of tensile strength. Ceramic catalytic support materials at high temperatures are brittle, undergo fracture propagation, and fragmentation. In large gas turbines used for electric power generation, the combustor element must have a diameter of about 21 inches. Under the conditions of operation, the catalyst support or honeycomb must not shatter or break loose.
P. W. Pillsbury et al in 1982 issued a report under the auspices of the Electric Power Research Institute, Report No. AP-2584, prepared by Westinghouse Electric Corp., Condorville, PA. Copies of this report are available from the Research Report Center, P.O. Box 50490, Palo Alto, CA 94303. This report deals with the effort to utilize a catalytic combustor in conjunction with a gas turbine. The combustor utilized ceramic monolith elements that were cemented together and comprised a 16" diameter combustor.
Commercially available ceramic monolith elements are cast or extruded with a cell density of 50 to 200 cells per square inch. The catalytic materials adhere to the walls of the ceramic and are typically applied by a process of dipping, in an aqueous solution, followed by calcining. Such catalytic ceramic honeycomb monoliths have been successfully used for periods of a few hours to reduce NO.sub.x to within the limits prescribed by the U.S. Environmental Protection Agency and the South Coast Air Quality Management District in Los Angeles. However, the ceramic support has proved to be fragile because of its relatively low toughness, low thermal conductivity, and low tensile strength in the face of the high temperature gradients throughout the combustor, especially during start up. At turbine start up, the temperature of the monolith can be elevated from normal outdoor temperature to over 2000.degree. F. within a few seconds.
Moreover, combustors made of ceramic have exhibited backflashing in which the air-fuel mixture in the fuel mixing chamber upstream of the combustor ignites, often with disastrous effects on the turbine system. Ceramic honeycomb cores have an area of approximately 70% open to the transmission of infrared energy. In the case of ceramic combustors, infrared heat energy travels in a straight line upstream through the tubular cells that are characteristic of a ceramic honeycomb. The result of this direct path of high temperature energy is that towards the upstream end of the combustor there exists an apparent temperature nearly as high as that of the combustion chamber, and sufficient to ignite the fuel. This explains why backflashing has been a problem with ceramic combustors.
The cell walls of such ceramic monoliths are from 0.006-0.025 inch thick. The individual monolith modules are typically 4 inches wide by 7 inches high with a depth as dictated by the combustor design. Typically, such depths are 8-12 inches. In order to make a large, homogeneous monolith for a combustor, the ceramic modules are cut to size on a diamond wheel and cemented together. The wall thickness is at least doubled along the seams where the blocks are cemented. When gases rush through the ceramic monolith at space velocities on the order of 1,000,000 volume/volume/hour, eddies are created at the entrance and exit of the catalytic chamber. The seams cause eddies and uneven flow which can induce backflashing from the hotter combustion zone through the catalytic converter and into the fuel mixing chamber, thus igniting the gaseous mixture of fuel and air in the fuel mixing chamber.
The present invention, on the other hand, utilizes mixed flow honeycomb cells characterized by a nonlinear flow path and which, by reason thereof, are opaque to infrared radiation while at the same time providing a relatively low pressure drop, diffusing conduit for the combustion gases as they traverse the combustor. Mixed flow honeycomb cells are cells which provide a tortuous path for the gas to traverse.
Reference may be had to Pfefferle's U.S. Pat. No. 3,846,979 dated Nov. 12, 1974; U.S. Pat. No. 3,928,961 dated Dec. 30, 1975; U.S. Pat. No. 3,940,923 dated Mar. 2, 1976; U.S. Pat. No. 3,975,900 dated Aug. 24, 1976; U.S. Pat. No. 4,019,316 dated Apr. 26, 1977; U.S. Pat. No. 4,094,142 dated June 13, 1978; and various catalytic systems and methods of making them as described in U.S. Pat. No. 4,276,203 dated June 30, 1981; U.S. Pat. No. 4,287,090 dated Sept. 1, 1981; U.S. Pat. No. 4,276,203 dated June 30, 1981; U.S. Pat. No. 4,295,818 dated Oct. 20, 1981; and U.S. Pat. No. 4,337,028 dated June 29, 1982. Up to this time, mainly extruded ceramic material impregnated with a suitable catalyst has been used for combustors.
Reference may also be had to U.S. Pat. No. 4,154,568 dated May 15, 1979 to Kendall et al which describes a catalytic combustor which includes a monolith bed through which a mixture of oxidant and fuel is directed and characterized by cells of optimum size for heterogeneous combustion and cells of smaller size downstream to obtain substantially complete combustion in a relatively compact bed. This has some tendency to opacity to infrared energy transmission.
There are currently no iron-based or nickel-based alloys that have been shown to withstand the combustor environment for prolonged service. Furthermore, many stationary turbine installations are required to demonstrate operation for at least about 15% of their life cycle with fuel oil which has sulfur concentrations ranging up to 60 ppm of SO.sub.2 in the exhaust stream. Sulfur dioxide, in the presence of oxygen, is a powerful oxidizer for metals at high temperatures.
Some attempts in the past to find suitable materials and practical methods of construction for combustors for gas turbine service are described in the foregoing patent literature.
The lack of suitable high temperature materials and a means for processing them on a large scale basis into suitable monolithic honeycomb cores has for some time been a fundamental obstacle in combustor technology for gas turbines. Catalytic combustors are generally recognized as the preferred means of eliminating thermal NO.sub.x because peak temperatures in the combustion chamber can be maintained at less than 3000.degree. F., above which thermal NO.sub.x is generated.
The program to develop a suitable combustor has become more urgent in view of the need to reduce NO.sub.x emissions from an increasingly large number of gas turbine power plants, and the relatively high cost of NO.sub.x reduction by means of selective catalytic reduction (SCR) and/or steam injection.
Power generation utilizing gas turbines has recently moved into center stage mainly because of (1) the favorable economics of gas turbine power plants relative to nuclear power and coal power plants, (2) the abundant supply of reasonably priced natural gas, and (3) the favorable field experience associated with over 800 land-based gas turbines in service for a number of years.
At the same time, in a number of states, NO.sub.x emissions from gas turbines are tightly controlled at 42 ppm maximum and in California at 9 ppm maximum. Low NO.sub.x limits are important because NO.sub.x combines with volatile organic compound emissions to form ozone. Ozone in the troposphere and especially in intermountain valleys, such as in the Los Angeles Basin and Mexico City, damages living matter. NO.sub.x is also indirectly responsible for acid rain in the Western United States, the toxic effects of which have been well documented.
It is a principal object of this invention to provide a catalyst member for use in such gas turbine catalytic combustors which is opaque to infrared radiation and highly permeable to the flow of an air/fuel mixture.
It is a further object of this invention to provide a catalytic member for use in such gas turbine catalytic combustors which is structurally stable and able to withstand the high temperatures necessary to operate the device efficiently, i.e., up to about 2700.degree. F.
It is still a further object of this invention to provide a device easily amenable to mass production and of lower cost than existing NO.sub.x control systems.
Other objects will appear as the description proceeds.